Beam deflection electrometer amplifier tubes



Feb. 12, 1957 3 Sheets-Sheet 1 Filed July 16, 1951 FIG.2.

INVENTOR N A M E L a nu H N H o J ATTORNEYS Feb. 12, 1957 J. H. COLEMAN BEAM DEFLECTION ELECTROMETER AMPLIFIER TUBES Filed July 16, 1951 5 Sheets-Sheet 2 az T 8 WM w W m E" 0 WE n .l ob A F c :7 H- y N H l, x w 4 V V 1 p. If I w. M 7 WW L 4 7 I a n I M l M; I L 6 6 F Feb. 12, 1957 J. H. COLEMAN BEAM DEFLECTION ELECTROMETER AMPLIFIER TUBES Filed July 16, 1951 3 Sheets-Sheet 5 INVENTOR rm ATTORNEYS FIG.9.

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JOHN H COLEMAN Unite BEAM DEFLECTION ELECggOMETER AMPLIFIER TUB John Howard Coleman, West Palm Beach, Fla, assignor V to Radiation Research Corporation, West EPaim Beach,

Fla., a corporation of Florida Application July 16, 1951, Serial No. 236,970

Claims. (Cl. 315-21) increased transconductance and high deflection sensitivity.

Previouspelectrometer amplifier tubes, as described in my copending application Serial No. 196,553, filed November 20, 1950, Patent No. 2,701,319 and entitled Electrometer Amplifier Tubes, provided for the detection and measurement of very small electric currents by tubes which deflected an electron stream in accordance with potentials applied to beam control electrodes, and accelerated said stream into collectors surrounded by an electric field-free space. This approach accomplished the trapping of positive ions (formed principally by collision of electrons with the inevitable neutral molecules of gas remaining in (or liberated in) the envelope after evacuation). For a complete discussion of this prior invention, reference may be had to said earlier application.

While the invention in accordance with said application did achieve its stated objects, it required special control apparatus to eifect a reduction in the flow of positive ions to the control electrodes, said positive ions being produced in the region of said control electrodes which were maintained more negative than adjacent electrodes with a view to obtaining a high deflection sensitivity.

In accordance with the present invention, on the other hand, a construction and a method of operation are provided which obviate the above difficulties. More specifically, the present invention deals with arrangements by which the electron beam of the tube is deflected after having been accelerated and focused on a suitable collecting member or members, the latter being associated with or efiectively surrounded by positive ion trapping means. Moreover, the invention makes unique provision for reducing the flow of current to the beam deflection control electrodes, and the novel combination of the ion trapping means (which operates to reduce the space charge in the electron beam) with the deflection control system results in an important increase in the transconductance obtainable with electrometer tubes of this type.

So far, the discussion herein has related simply to effective sensing or measurement of very small currents, but the attainment of this general result also makes possible a substantial improvement in other applications of such tubes; for example in high frequency amplification, signal mixing and like'functions, as will appear more fully as the description proceeds.

It is therefore a principal object of my invention to provide a novel arrangement of the electrodes of an electrometer tube of the beam-deflection type to accomplish improvements in the sensing and measuring of very small currents.

A further object of my invention is to provide an improved method of operating a tube of this type to achieve increased deflection sensitivity and high transconductance, both for measurement and amplification purposes.

Another object of my invention is to provide animproved electrometer tube operable as an extremely sensitive signal mixer at high frequencies.

Still another object is to provide improvements in the design and operation of cathode ray tubes, particularly with reference to cathode ray electrometer tubes, for either electrical or optical outputs.

The above and other objects and advantages of my invention will best be understood from the following detailed specification of certain preferred embodiments thereof, and the preferred manner and method of practicing the invention, reference being made to the accompanying drawings, in which:

Fig. l is a perspective View, with parts broken away and parts in section, of one form of electrometer tube according to the invention,

Fig. 2 is a longitudinal cross-section of the same embodiment,

Fig. 3 is a view similar to Fig. 2 but of a modified form embodying a special form of ion trapping grid, and arranged to give a visual output,

Fig. 4 is a similar view of another form of the invention having a beam limiting aperture acting as an object (in the optical sense) for the following electron lens,

Fig. 5 is a longitudinal cross-section of a further modification in which the electron lens is independent of the deflection control electrode potential,

Fig. 6 is a similar view of a form in which the deilection electrodes are disposed in the ion trapping re glon,

Fig. 7 is a similar view of a structure combining features of Figs. 5 and 6, and providing a plurality of sets of deflection electrodes spaced along the path of the beam,

Fig. 8 is another similar view of a form of the invention employing a visual output or indication (applicable to any of the other forms of electrode arrangements) and having both horizontal and vertical deflection plates for deflecting a beam of circular cross-section, and

Fig. 9 is a transverse section taken on line 9-9 of Fig. 8. 7,

Referring now to Figs. 1 and 2 of the drawings, there is shown a beam deflection electrometer tube 10 comprising an evacuated envelope 12 which may be of glass or other suitable material in which are mounted the electrode elements to be described. If a metal envelope is employed, suitable insulation for the connecting leads will be provided as will be understood by those skilled in this art. For purposes of clarity and simplicity, the drawings do not illustrate conventional means for supporting the electrodes in their proper positions; in some cases, the connecting leads themselves can perform this function, and auxiliary supports can readily be provided Where required.

The source of the electrons for tube 10 preferably is constituted by a cathode structure 14 which may,com prise a standard emitting material 16 such as a mixture of barium. and strontium oxides contained in a cup 18 having a rectangular front or emitting face. The cathode 14 is indirectly heated to incandescence by a resistance heating element 20 of known form, the same being connected to a suitable power supply through leads 22. A cathode connection is indicated at 24, and the electric field at the emitting face of the cathode is controlled by an electrode 26 in the form of a rectangular metal sheet 28 having a rectangular aperture 30 concentric with the emitting face. A connection for applying potential to electrode 26 is designated by numeral 32.

Electrons emitted from cathode 14 are formed into a divergent beam (34 in Fig. 2) by an accelerating field applied between the cathode and an electrode 36, the connection to this last electrode being shown at 38; Electrode' 36 is also formed as a rectangular sheet or plate having a concentric rectangular aperture 40.

Beyond electrode 36, the beam passes betweena pair of deflecting electrodes 42 and 44, each being of rectangular shape and disposed parallel to the central axis of the beam on opposite sides thereof. Connecting leads 46 and 48 for these electrodes are supplied with potentials establishing between the plates an electric field which'will converge the electron stream (again as indicated by dash lines in Fig. 2) when the applied potential is less than that applied to electrode 36. For brevity, the potential applied to a particular electrode will hereinafter be referred to by the letter V followed by the numeral designating' that electrode; e. g., V36.

From deflection plates 42 and 44, the still-converging beam passes into the positive ion trapping region designated 50 and formed by spaced conductively connected sheets 52 and 54 and lateral sheets 56. A rectangular aperture 58 in sheet 52 has a grid formed of transverse conductive rod's or wires 60, which grid defines with the other elements connected thereto an equipotential space for ion trapping. The depression of the potential of the beam resulting from its space charge draws in a s'ufiicient number of positive ions to neutralize the space charge, these ions resulting from ionizing collisions of the electrons of the beam with neutral gas molecules. The result of this space charge neutralization is a higher current density of the beam, and consequently higher transconductance characteristics for the tube. An additional result of this ion trapping region is that positive ions therein cannot reach the deflecting electrodes 42 and 44, and positive ion currents to these electrodes are therefore decreased. While positive ions formed between the grid 60 and electrode 36 could reach the deflecting electrodes, this flow can be minimized by lowering the potential V60 of the grid (that is, the potential applied to lead 62) as close as possible to V42 without producing objectionable defocusing of the beam. A

The focused electron beam ultimately strikes the center of end plate 54 (this center point being indicated by a dot 64 in Fig. l), and is deflected vertically by the application tothe deflection electrodes of a potential derived from the minute" current to be sensed or measured. The vertical motion will cause the beam to strike or overlap a collecting electrode 66 having a connecting lead 63, and thereby produce an output signal in an external load circuit in-the usual way. Collector 66 is shown as disposed within an aperture formed off-center in plate 54, but it can equally well be disposed beyond such an aperture as shown in other views of the drawings. The degree to which the beam is' deflected onto collector 66 deter: mines the output signal in the load circuit; The modified form shown in Fig. 3 of the drawings is generally similar to that just described (like parts being indicated by the reference numerals used above), but is arranged to provide a visual, rather than an electrical, indication. To this end, the end plate 54 of Figs. 1 and 2 is replaced by a transparent sheet 70, having a coating or layer 72 of a suitable phosphor such as willernit'e, and a known aluminized backing 74 may be provided to prevent charge formation oh the phosphor or its support sheet 74 by secondary emission and sweeping out of positive ions. v

Fig- 3 also shows a modification of the grid; this ele-' ment 76 being formed convex toward the direction of beam flow, which reduces the defocusing effect produced whenV76 approaches the value of V42; V44. simi- 4 larly configured grid may of course be utilized in the structure of Figs. 1 and 2.

Fig. 4 shows a modified form of the invention utilizing a more elaborate electron lens system. In this modification, the plates 78 and 80 are connected to lead 82 (corresponding to lead 38 of Figs. 1 and 2), and each of these plates has a beam limiting aperture 84 and 86 respectively. Aperture 84 acts as a sharp edged object to be imaged on the collector plates 88 which correspond to collector 66 of Figs. 1 and 2. The second aperture e26 also acts to limit the beam, cross plates 90 connecting these two plates with electrode 92 which corresponds'to electrode plate 36 of the forms described above. This use of a multiple aperture enables the potential applied to deflection electrodes 42 and 44 to be raised above those applied to electrodes 92 and 52, while still obtaining beam convergence, and positive ions will be repelled from the deflection electrodes. In this modification, also, the grid 94 is made concave in the direction of beam flow, which will focus the beam in the decelerating field.

The collector plates 88 in Fig. 4 are placed behind a rectangular aperture 96 in end plate 98in order to reduce any effect of positive ion attraction by the collector plates when the potential on any of them drops as a result of its collection of electrons from the beam. This displacement of the collectors as contrasted with the coplanar at rangement of Figs. 1 and 2 also provides an overlapping intercepting edge at the aperture 96. The use of several spaced-apart collectors permits discrimination between defiection control potentials of different magnitudes and polarities.

In the form of the invention shown in Fig.- 5, a' third type of beam accelerating structure is shown, including" rectangular apertured electrodes 99 and 100, and an electrode 102 corresponding to 36 of Fig. 2. When voltages are applied to these three electrodes (via their respective connections), a converging lens can be produced with V99 equal to V102 and greater than V100, this lens being independent of the deflection electrode voltage V42, V44. End plates 104 are provided to connect electrode 102 to electrode 106 (corresponding to 52 of Fig. 4) and to plate 108' corresponding to 98 of Fig. 4, with plates 42 and 44 between 141-2 and 106, so that the potentials at 42',- 44' are equalized for minimum internal current.

Fig. 5 also represents a modified collector system having a plate 108 whose aperture 110 is transversed by a horizontal conductive rod or wire 112'. This rod inter cepts the beam at zero signal, and the (in effect) double aperture or pair of apertures provides output signal cur-' rent for both positive and negative signals.

Fig. 6 of the drawings illustrates an embodiment similar to Fig. 5, but with the deflecting electrodes 42, 44 disposed within the ion trapping region; that is, between plates 114 and 116. With this arrangement, the voltages on 42 and 44 are adjusted to be equal, so that there is'no posi-' tive ion attraction at zero input or control signal. The magnitude of the positive ion current attracted by 42 will depend upon the magnitude and frequency of the applied signal. The higher the frequency and the lower the magnitude of the signal, the smaller will be the ion current due to ion mobility. This modification is therefore es pecially adaptable for high'frequency amplification.- I

The lens system of Fig. 6 is similar to that of Fig; 5, but with electrode 1-14 replacing electrode'102 of Fig.- 5, and the concave grid 94 again assists in focusing when V100 is greater than V114. v 4

Fig. 7 shows an embodiment in which two independent sets of deflection electrodes are used, permitting tit-ease trol of the output to be exercised for applications such as signal mixing. As in Fig. 3, one set of defieet'iofi lc trodes is disposed in the trapping region (electrodes 1'18, and the other set 122, 124' between the electrode 126 (whose aperture is covered by convex grid 128)" and the electrode 130. Respective connections for applying signals to these deflection electrodes are indicated the drawing, but do not require further reference as they operate in a manner which will be clear from the description of preceding figures. The convex grid 128 prevents aberrations in focusing when V118, 120 is equal to V130.

The collector system in Fig. 7'is identical with that shown in Fig. 5, and has therefore been given the same reference numerals, but needs no further description. This form of the invention, due to the considerations explained above in connection with Fig. 6, is especially eflicient at higher frequencies, and can therefore be ap plied in ultra-high-frequency mixing.

Figs. 8 and 9 show in longitudinal and transverse sectional views a form of the invention in which the electron beam can be deflected in two orthogonal directions. In this modification, the square electrode plates 132 and 134 have circular, rather than square, beam apertures, so as to form a beam of circular cross-section. Deflection plates 136, 138 provide for vertical deflection, and plates 140, 142 (best seen in Fig. 9) for horizontal deflection. The lead 144 serves as a common connection for electrodes 132, 134 and 146, which are connected by end plates 148, which also connect electrode 146 with the aluminized backing (if used) on the phosphorescent end screen 150. The operation of such a screen to provide a luminous or visual output will be clear in view of the previous description respecting Fig. 3.

The trapping electrode 146 is shown in elevation in Fig. 9, and as shown has a square aperture and concave grid 152. However, in view of the circular beam crosssection employed in this form, this aperture could equally well be circular (as in electrodes 132 and 134), in which case the grid would have the profile of a segment of a sphere.

While the invention has been described herein in connection with centain specific embodiments for purposes of illustration and to comply with the patent statutes, it is to be understood that modifications can be made both in the suggested forms of apparatus and in the method disclosed, by those skilled in this art, without departing from the spirit of the invention as defined in the appended claims.

Iclaim:

1. An electrometer tube comprising an evacuated envelope, a source of cathode rays in said envelope, means defining a substantially field-free space and including transverse members forming a transverse gridded aperture, means for accelerating electrons from said source and directing them as a beam through said gridded aperture into the substantially field-free space to trap positive ions, deflecting electrodes intermediate said accelerating means and said trapping region for establishing a deflecting field, said deflecting electrodes being disposed parallel to the central axis of the beam on opposite sides thereof, and means for collecting electrons after deflection of said beam.

2. The invention in accordance with claim 1, in which said collecting means comprises an electron collecting electrode for connection with a load circuit, to provide an electrical output signal.

3. The invention in accordance with claim 1, in which said collecting means comprises a fluorescent screen to provide a visual or optical output signal.

4. A cathode ray tube for sensing and measuring minute currents, comprising an evacuated envelope, 2. source of electrons therein, means for accelerating and focusing said electrons into a beam, beam deflection electrodes adjacent the path of said beam disposed parallel to the central axis of the beam on opposite sides thereof, an ion trapping electrode including an aperture, an ion trapping grid including transverse members forming a grid having a contour like a portion of a spherical surface, concave with respect to the deflection electrodes adjacent and connected to said trapping electrode over said aperture, an electron collector, and means for defining a substantially field-free space between said grid and said collector.

5. The invention in accordance with claim 4, in which the potential of said ion trapping electrode is maintained substantially at the value of potential applied to said beam deflection electrodes.

6. The invention in accordance with claim 4, in which said beam deflection electrodes are so energized as to cause a fall in the potential of said beam, whereby to neutralize space charge therein by the drawing in of positive ions and thereby to lower the current flow to said beam deflection electrodes.

7. An electron tube for the sensing and measurement of very small currents and characterized by high transconductance and high deflection sensitivity, comprising an evacuated envelope, a source of electrons therein, means for accelerating and focusing electrons from said source into a beam, deflecting electrodes spaced laterally of the path of said beam and parallel thereto, an ion trapping electrode including a gridded aperture of transverse members beyond said deflecting electrodes, a collector for said beam, and means for defining a substantially fieldfree space between the apertured grid and the collector.

8. The invention in accordance with claim 7, in which said grid has a curved configuration relative to the direction of the beam.

9. The invention in accordance with claim 7, in which said collector comprises a plate disposed laterally of the direction of the undeflected beam.

10. The invention in accordance with claim 7, in which said collector comprises a plurality of plates spaced in a direction transverse to the direction of said beam.

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